1. Field of the Invention
This invention relates to the field of vapor-phase deposition. This invention further relates to susceptors for crystal growth of epitaxial films on substrates. The invention also relates to a truncated susceptor for hydride vapor-phase epitaxy systems. This invention still further relates to a method for vapor-phase deposition in which premature deposition on a susceptor is avoided.
2. Backaround of the Related Art
Hydride vapor-phase epitaxy (HVPE) remains an important technique for the epitaxial growth of various semiconductors, such as gallium nitride (GaN). In HVPE systems, growth proceeds due to the high temperature, vapor-phase reaction between gallium chloride (GaCl) and ammonia (NH.sub.3). The two gases are directed towards a heated substrate where they meet and react to produce solid GaN on the substrate surface. There are, however, certain difficulties associated with this growth technique. For example, the source or reagent gases can react before reaching the substrate, leading to premature deposition of GaN, i.e. GaN deposition on non-target surfaces. The substrate typically rests on a susceptor which is arranged at an angle with respect to the direction of gas flow (e.g., refer to FIG. 2). The entire susceptor, and not just the substrate, is maintained at the elevated temperatures necessary for deposition to occur. Thus, growth of solid GaN can occur on the susceptor upstream from the substrate. Such upstream GaN deposits have negative consequences for crystal growth on the substrate. Firstly, deposits of solid GaN tend to obstruct proper flow of reagent gases towards the substrate. Furthermore, if the unwanted GaN deposits accumulate beyond a certain thickness, they tend to merge with the epitaxial layer that is the object of the deposition. Merger of unwanted GaN deposits with the epitaxial layer degrades the uniformity of the GaN epitaxial layer and leads to an inferior product.
Existing methods of dealing with unwanted deposits in HVPE systems generally involve periodic removal of the susceptor, before depositions, from the reaction chamber to allow cleaning of the susceptor, either physically or by chemical etching. Such methods, however, require frequent interruption of reactor use, and are labor-intensive, time-consuming and hazardous. Furthermore, in situations where it is desired to deposit a thick layer of GaN on a substrate, unwanted deposition on the susceptor can reach problematic levels during the course of a single growth cycle. However, cleaning and maintenance of a HVPE system can only be performed after a growth cycle is complete. Techniques that attempt to remove unwanted deposits in situ, such as by passing through an etchant gas, face similar limitations: system etching also requires interruption of reactor use, and cannot be performed during a growth cycle. Also, any etchant gas that etches deposits in the susceptor will also etch the epitaxial film.
The present invention resolves problems associated with the premature or non-target growth of crystals in prior art vapor-phase chemical deposition systems and methods. The present invention minimizes labor-intensive, time-consuming, and expensive maintenance of a HVPE system by preventing premature upstream deposition on the susceptor surface. In particular, the invention eliminates problems associated with unwanted deposition on susceptor surfaces upstream from the substrate during hydride vapor-phase epitaxy.